The invention relates generally to marine seismic prospecting and, more particularly to apparatus and methods for reducing the effects of spurious seismic reflections in hydrophones arrayed in a streamer towed behind a survey vessel.
In marine seismic exploration, a hydrophone array is towed behind a marine vessel near the sea surface. The hydrophones are in multiple linear hoses known as steamers. A seismic source, also towed near the sea surface, periodically emits acoustic energy. This acoustic energy, which is in the seismic frequency band, travels downward through the sea, reflects off underlying rock structures, and returns upward through the sea to the hydrophone array. Ideally the hydrophone array records the upward traveling seismic acoustic wave from the seabed. The hydrophone recordings are later processed into seismic images of the underlying rock structures.
Because a hydrophone has an omni-directional response, the hydrophone array also records a ghost response, which is the desired seismic acoustic wave reflected from the sea surface and arriving delayed in time and reversed in polarity. The ghost is a downward traveling seismic acoustic wave that, when added to the desired wave, blurs the recorded seismic image. A similar visual effect occurs with broadcast television using an old-fashioned “rabbit ear” antenna.
The ghost produces a notch in the frequency spectrum of a hydrophone record at fnotch=c/2d, where c is the speed of sound and d is the streamer depth. Seismic streamers have been conventionally towed at a depth of 10 meters. At d=10 m, fnotch=75 Hz. A frequency response extending beyond 100 Hz is required for high seismic image resolution. At d=4 m, fnotch=188 Hz. Streamers are therefore towed at a depth of 4 meters to improve the resolution of a seismic image. But towing at 4 meters increases downtime due to adverse weather and accompanying high sea states. Furthermore, seismic image quality would actually improve at greater towing depths because there is less acoustic background noise at greater depths and because the auxiliary equipment used to measure and control the hydrophone positions works better at greater depths for a given sea state.
Thus, there is a need for towing a streamer at any practical depth with high seismic image resolution to increase the productivity of seismic surveying.
Ocean-bottom systems, in which the seismic streamer is laid on the seabed, reject ghosts by a technique known as p-z summation. In an acoustic wave, the pressure p is a scalar and the particle velocity u is a vector. A hydrophone records the seismic acoustic wave pressure p, with a +omni-directional response. A vertically oriented geophone, often implemented with an accelerometer, records the vertical component of the seismic acoustic wave particle velocity uz, with a figure-of-8 response, +lobe pointing down and −lobe pointing up, as illustrated in the beam patterns of FIG. 10. In p-z summation the velocity signal is scaled by the acoustic impedance pc of seawater and added to the pressure signal. This produces a compound sensor that has full response to the upward traveling wave and zero response to the downward traveling wave to reject the ghost.
Ocean-bottom streamers experience any roll angle from 0° to 360° and moderate pitch angles. To implement a vertically oriented geophone, ocean-bottom systems have used: (a) a gimbaled moving-coil geophone; (b) a 3-component, omni-tilt moving-coil geophone with attitude sensing and synthetic uz computation external to the sensor; and (c) a 3-component, micro-electro-mechanical system (MEMS) accelerometer geophone with internal attitude sensing and synthetic uZ computation external to the sensor. But all these solutions have shortcomings, such as large size, mechanical reliability, and reliance on external computation.